As to an ignition device of an internal combustion engine (hereinafter also referred to as an engine), an ignition device of a type that an ignition plug is arranged in a head cover of the engine such that an ignitor directly protrudes into a combustion chamber and a high voltage generated in an ignition coil is applied to the ignition plug has been known.
For example, as shown in FIG. 8, the ignition plug used in such an ignition device includes an insulator 12 held by cylindrical mounting hardware 11, a central electrode 13 held within insulator 12 and having a tip end protruding from the end of insulator 12, and a ground electrode 14 opposed to central electrode 13 with a prescribed spark gap Ga being interposed, and its structure is such that spark discharge is generated between central electrode 13 and ground electrode 14 by applying a high voltage across central electrode 13 and ground electrode 14.
Meanwhile, depending on an operation state of the engine, deposit may be formed on the ignition plug. The deposit on the ignition plug refers to such a phenomenon that carbon originating from incomplete combustion or the like in the engine adheres to the insulator of the ignition plug, which causes lowering in an insulation resistance value of the ignition plug. When a degree of such deposit on the ignition plug develops, a leakage current flows between the ground electrode and the central electrode of the ignition plug as a result of application of a high voltage at the time of ignition, a voltage across the electrodes lowers, and spark discharge does not occur, which may lead to misfire.
In order to suppress deposit on the ignition plug, conventionally, (1) devising a shape of the ignition plug (for example, a method of promoting deposit cleaning by shaping the ignition plug such that spark sweeps over the top surface of the insulator), (2) a method of cleaning deposit by generating creeping discharge over the top surface of the insulator by using an auxiliary electrode, (3) a method of cleaning deposit by using multiple discharge, and the like have been performed.
Japanese Patent Laying-Open No. 2002-161841 proposes, as another method of suppressing deposit, a method of eliminating deposit by controlling a motor-generator so as to increase electric load imposed on an internal combustion engine and to increase a temperature in a combustion chamber when the deposit is formed on the ignition plug in the internal combustion engine where the motor-generator is operatively coupled.
As to a method of detecting a degree of deposit on the ignition plug, for example, a method of detecting the degree of deposit, by applying a voltage across electrodes of the ignition plug (across the ground electrode and the central electrode), detecting a current that flows between the electrodes (leakage current) with a current detection device, and estimating lowering in an insulation resistance value based on the detected current value, is available.
The deposit formed on the ignition plug can be suppressed with the cleaning method as described above. In the ignition plug, however, in addition to carbon, a metallic additive in fuel (such as iron or manganese) adheres to the insulator and conductive deposit is formed. As such conductive deposit cannot be self-cleaned, it is necessary to issue a warning or the like to a driver at an early stage in order to avoid failure in running.
Meanwhile, the deposit composed of carbon or the like is also conductive accretion. Accordingly, even if the leakage current is detected by the current detection device above, it is impossible to determine whether the leakage current is caused either by the deposit or by the conductive deposit, or whether it is caused by both of the deposit and the conductive deposit. Therefore, it is difficult to take measures for issuing a warning to the driver, of adhesion of the conductive deposit to the ignition plug, and failure in running due to the conductive deposit is concerned.